Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision. A human eye typically comprises three types of cones, each of which has a response curve (roughly a normal distribution) over a range of wavelengths of light and a peak sensitivity over a particular, smaller range of wavelengths of light. Long wavelength sensitive cones, also referred to as L cones or red cones, respond most intensely to light having long wavelengths: the peak sensitivities of red cones are typically around wavelengths 564-580 nm (greenish-yellow light). Medium wavelength sensitive cones, also referred to as M cones or green cones, respond most intensely to light having medium wavelengths: the peak sensitivities of green cones are typically around wavelengths 534-545 nm (green light). Short wavelength sensitive cones, also referred to as S cones or blue cones, respond most to light having short wavelengths: the peak sensitivities of blue cones are typically around wavelengths 420-440 nm (blue light).
Myopia is a refractive defect of the eye in which light entering the eye produces image focus in front of the retina, rather than on the retina itself. Myopia is often colloquially referred to as nearsightedness. Myopia may be measured in diopters, which is a unit of measurement of the optical power of the eye's lens, equal to the reciprocal of the focal length of the lens.
Television, video games, and computer monitors all cause progression of myopia in children because those displays produce stimuli that cause uneven excitation of the red and green cones. Differential activation of the red and green cones is responsible for the eye elongating abnormally during development, which in turn prevents images from being focused clearly on the retina. Unnatural red stimuli such as those from cathode ray tube (CRT) displays preferentially activate red cones six to eight times more effectively than green cones. The red primaries in CRTs have been matched by all modern display types (liquid crystal displays (LCD), displays that combine liquid crystal displays with light-emitting diodes (LCD/LED), organic LED displays, plasma displays, and digital light processing displays (DLP), to name a few), making virtually all electronic displays prone to causing myopia.